1. Field of the Invention
Embodiments of the present invention relate to geophysical surveys, methods of processing seismic data, and, in particular, to use Wavefield Extrapolation Multiple Modeling (WEMM) to predict multiples for seismic data recorded by dual sensors on the ocean bottom cables, so as to attenuate multiples.
2. Description of the Related Art
The following descriptions and examples do not constitute an admission as prior art by virtue of their inclusion within this section.
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying may typically utilize seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. The seismic waves may propagate into the formations in the earth, where a portion of the waves may reflect from interfaces between subterranean formations. The seismic receivers may detect the reflected seismic waves and convert the reflected waves into representative electrical data. The seismic data may be transmitted by electrical, optical, radio or other means to devices which record the data. Through analysis of the recorded seismic data (or seismograms), the shape, position and composition of the subterranean formations may be determined. This is mostly used to indicate the presence or absence of probable locations of hydrocarbon deposits.
Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying may typically utilize seismic energy sources and seismic receivers located in the water which may be either towed behind a vessel or positioned on the water bottom from a vessel. The energy source may typically be an explosive device or compressed air system which generates seismic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water.
A seismic wave travels from a source, reflected once by an interface of subsurface formations and received by a receiver, is a primary (P). The signal it generates is the desired primary signal. A seismic wave may also travel through other routes, be reflected multiple times at various interfaces before reaching a receiver. These waves generate multiples. Depending on which additional interfaces reflect the seismic wave, the multiples may be called Surface related multiples, internal multiples etc. Except the primary signal, all signals (mostly multiples) generated at the receivers by other waves are considered noises and need to be removed from the recorded data.
Various methods have been developed to attenuate multiple reflections in seismic data. For example, Surface Related Multiple Elimination (SRME) is a process that predicts surface multiples by stacking convolved pairs of recorded seismograms and adaptively subtracting the predicted multiples from the recorded seismograms to remove surface multiples. However, application of SRME may require awareness of several possible problems. Two potential problems in using SRME are properly handling ghosts and properly predicting sea surface reflection effects.
Some have proposed the use of WEMM to predict the surface multiples that appear in marine streamer seismic data. Because Ocean Bottom Cable survey is quite different comparing to the towed streamer seismic survey, those methods applicable to towed streamer data are not directly applicable to OBC data. There are at least two major differences between streamer surveys and ocean-bottom cable (OBC) surveys. Firstly, in OBC the wavefield sensors are located on the ocean bottom, while the sources are usually in water body at a different depth. In OBC survey, sources and receivers are at different depth, unlike in towed streamer survey, the sources and streamers are at almost the same depth. Secondly, two types of sensors (pressure measurement, e.g. hydrophones and displacement measurement, e.g. geophones) are typically used in OBC as compared to the standard single sensor type (hydrophones) used in streamers. To account for and take advantage of these differences, OBC data typically require different methodology.
One method for OBC data is considered in the Pica, et al., (2006) reference. The method assumes that the sources are located at the sea surface and that the up-going wavefield at the ocean bottom is available. Reciprocity is invoked and the data are sorted into common-receiver gathers. This approach allows the method for marine streamer data to be used. In other words multiples are predicted by extrapolating the wavefield from the sea surface through one traverse of the subsurface.
Here are some references discussing WEMM:
Pica A., G. Poulain, B. David, M. Magesan, S. Baldock, T. Weisser, P. Hugonnet, and P. Herrmann, 2005, 3D surface-related multiple modeling, principles and results: 75th Annual International Meeting, SEG, Expanded Abstracts, 2080-2083.
Stork, C., Kapoor, J., Zhao, W., Dragoset, B., and Dingwall, K., 2006, Predicting and removing complex 3D surface multiples with WEM modeling—an alternative to 3D SRME for wide azimuth surveys?: SEG Expanded abstract.
Pica, A., Manin, M., Granger, P. Y., Marin, D., Suaudeau, E., David, B., Poulain, G. and Hermann, P. H., 2006, Using and Removing 3D Surface-Related Multiples from OBS data. (proceedings of EAGE Workshop).
There is still a need for a proper method to attenuate multiples in OBC surveys. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.